This is a very simple engine to build and is made so because it has no valves. On the upward (power) stroke the piston seals but on the downward stroke the piston rocks in the cylinder causing it to leak and therefore coast for one half cycle.
Above you’ll see that I’ve added a couple of magnets and a small coil to the engine to create a generator. These are not described in the basic plans supplied.
The two 1/8”diameter super magnets are installed with opposite polarities facing out. The coil was salvaged from a very tiny relay. The arrangement generates alternating current. The two LED’s are arranged back-to-back so they flash on alternate cycles. No current limiting resistors are required because the coil has a winding resistance of about 5000 ohms. In the video above the LED’s appear to flash erratically. This is because of the frame rate of the video camera compared to the speed of the engine.